Polypropylene resins are inexpensive in general and are widely used as industrial materials for automobile parts and electrical and electronic parts and a variety of packaging materials utilizing properties such as lightweight, chemical resistance, acid resistance, thermal resistance, and so forth.
In the field of packaging materials, the polypropylene packaging materials in particular require high functionality in order to cope with environmental problems, and improvement in characteristics of the polypropylene packaging materials is strongly demanded also from the viewpoint of cost competition.
However, the polypropylene resins have a disadvantage of being inferior in moldability such as extrusion molding, foam molding, blow molding, and so forth. For example, a method of allowing an organic peroxide to react with a crosslinking additive in a resin of a melted state (Japanese Patent Laid-Open No. 93711/1984, Japanese Patent Laid-Open No. 152754/1986, and so forth), a method of allowing a peroxide having a low decomposition temperature and added beforehand to react in an inactive atmosphere (Japanese Patent Laid-Open No. 298536/1990, and a method of heat treating by irradiating a resin with electron rays in an decomposition atomosphere (Japanese Patent Laid-Open No. 121704/1987), and so forth have been proposed as methods for solving this problem.
These conventional methods each improve part of characteristics of the polypropylene resins, but are insufficient in melt tension, leave still a defect of lowering the melt tension when the resins are re-melted for recycling. In addition, the method of using an organic peroxide has the disadvantages of causing odors because of use of a crosslinking additive, terribly lowering the melt tension after being re-melted, and causing coloring with formulations of additives.
The inventors disclose resins having high melt tension and good recycling properties by use of polypropylene resins having high stereoregularity and a low MFR and irradiated with ionization rays in Japanese Patent Laid-Open No. 12761/1997 and Japanese Patent Laid-Open No. 104789/1997. These resins having undergone the ray treatment may be stored or transported for further processes for a certain time and often exposed to a high temperature of 40° C. or above. For example, the resins irradiated in an atmosphere of air show phenomena of rise in the MFR and decrease in the melt tension when allowed to stand at a temperature of 40° C. or above for a long time. In order to solve these problems, a method of storing at 40° C. or below after irradiating with ionization rays, a method of storing in an atmosphere of nitrogen, or a method of heat treating in an atmosphere of nitrogen or under vacuum after irradiation makes the improvement possible, but each has problems in costs and quality control. However, it was found that by use of the compositions of the invention after an ionization-ray irradiation treatment or a peroxide treatment, the change of the MFR is small, and the lowering of the melt tension also is hardly observed even after allowing to stand in an atmosphere of air at 50° C. or above for a long time.
Thus, the invention aims at solving the problems of related art as described above and providing a polypropylene resin composition high in the melt tension, causing no coloring, and small in lowering of the melt tension when recycled.